Process of back etching a perforated metal mask for color tv tubes

ABSTRACT

A 23-INCH RECTANGULAR SHADOW MASK COLOR TUBE IS SCREENED, USING THE APERTURES OF THE MASK IN DETERMINING   THE POSITION AND DIMENSIONS OF THE PHOSPHOR DOTS, AND THEN THE MASK IS RE-ETCHED TO ENLARGE THE DIAMETER OF ITS APERTURES, RELATIVE TO THE PHOSPHOR DOTS, TO A VALUE AO. THE THICKNESS T OF THE MASK AND THE PARAMETERS OF THE RE-ETCH ARE DETERMINED SO THAT THE QUANTITY T3-A0 HAS A VALUE OF APPROXMATELY 13 OR GREATER.

United States Patent O 3,679,499 PROCESS OF BACK ETCHING A PERFORATED METAL MASK FOR COLOR TV TUBES Thomas C. Minzenberger, Oak Lawn, Ill., assignor to Zenith Radio Corporation, Chicago, Ill. Filed Jan. 30, 1970, Ser. No. 7,281 Int. Cl. C23! 1/02, 17/00 US. Cl. 156-7 2 Claims ABSTRACT OF THE DISCLOSURE A 23-inch rectangular shadow mask color tube is screened, using the apertures of the mask in determining the position and dimensions of the phosphor dots, and then the mask is re-etched to enlarge the diameter of its apertures, relative to the phosphor dots, to a value A The thickness T of the mask and the parameters of the re-etch are determined so that the quantity T /A has a value of approximately 13 or greater.

BACKGROUND OF THE INVENTION The subject invention concerns processing of shadow mask color picture tubes which feature what has come to be known as re-etch. In accordance with such screening techniques, the shadow mask is originally etched and formed with apertures that are smaller in size than required for the mask as finally installed in the color tube but are dimensioned appropriately to use the mask in locating and dimensioning the phosphor dots of the screen. After screening has been accomplished, the mask is etched a second time, that is to say, it is re-etched to attain a hole size that is larger by a predetermined amount than the individual phosphor deposits. This screening technique is useful in the fabricating of different types of color tubes, especially those which feature black surround or post-deflection acceleration.

A black surround tube of a preferred form is the subject of Pat. 3,146,368, issued Aug. 24, 1964 in the name of Joseph P. Fiore et al. and assigned to the assignee of the present invention. It differs from conventional shadow mask types of cathode-ray tubes in two material respects: (1) each phosphor dot of the screen is surrounded by a material, such as graphite, that is absorptive of light, and (2) its electron beam is larger than the diameter of the phosphor dots. A post-deflection-focus or acceleration color tube differs from the conventional shadow mask device in that additional beam focusing is accomplished between the center of deflection and the screen. Because of the added focusing, more of the beams impinge upon the screen than otherwise and it is necessary, because of the post-deflection focusing, that the phosphor dots be smaller than the apertures of the mask. As stated above, in this respect, it is similar to the black surround tube.

The concept of establishing apertures in the shadow mask of a relatively small size to accommodate the mask for use in printing of the screen and then enlarging the apertures after screening and before final installation of the mask into the tube is indeed well known. In prior proposals, the mask blank is originally etched to have apertures of a desired final size and the apertures are then temporarily filled with a removable material in order to step down the aperture size for screening purposes. As a matter of practice, this approach is found to have serious difiiculties including, among others, non-uniformities of the phosphor dots obtained in the screening process.

In a preferred approach, the technique of re-etching or etch back as described above is practiced because it affords a materially greater degree of precision in dimensioning the phosphor dots of the screen, whether they be of uniform or graded size, and also distinctly superior results from the standpoint of uniformity of the phosphor deposits. While these advantages are attractive, still further improvement is desired so that, as taught by the subject invention, there may be attained an optimum relation of final aperture size and mask thickness to assure adequate mechanical strength of the shadow mask.

Accordingly, it is an object of the invention to provide an improved re-etch process for use in fabricating color cathode-ray tubes.

It is a specific object of the invention to improve the re-etch process of color cathode-ray tube fabrication in respect of the mechanical strength of the shadow mask.

SUMMARY OF THE INVENTION In processing a color cathode-ray tube, the mask is used to print on the screen patterns of different phosphor materials individually comprising phosphor dots corresponding in number and having a diameter approximately equal to, and arranged in the same pattern as, the apertures of the mask. Thereafter, the mask is etched to increase the diameter A of its apertures over the diameter of the phosphor dots. The invention concerns the improvement in such a process which comprises dimensioning the thickness T of the mask and determining the parameters of the etching step to obtain for the quantity T /A a minimum value which occurs at the knee of a curve relating that quantity to the mechanical strength of the etched mask which curve is non-linear and approaches a predetermined larger value asymptotically for values of the quantity above the aforesaid minimum value.

DESCRIPTION OF THE DRAWING The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals identify like elements, and in which:

FIGS. 1, 2 and 3 are curves utilized in explaining properties of the re-etch process and the improvement contributed by the subject invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT While the invention is useful in processing different types of color cathode-ray tubes, for convenience, it will be described in particular relation to a tube of the black surround variety. The general processing of such a tube is disclosed in the above-identified Fiore et al. patent which discloses two basic procedures, either of which may be employed. One has the choice, for example, of first depositing the dots of various phosphor materials over the screen area of the tube and then surrounding those deposits with a material that has light absorbing properties. Alternatively, a coating of light absorbing material may be applied to the image area of the tube and, thereafter, screening of the phosphor materials is undertaken to deposit phosphors in carefully developed openings of the coating of light absorbing material. Since the subject invention is addressed most particularly to improving the re-etch or etch back step, for the case in which it is necessary to enlarge the apertures of the mask after screening has been accomplished, the screening itself will be considered only briefly and, will be described in relation to the procedure in which the phosphor dots are applied first. This is also appropriate since the description then will be largely generic to the fabrication of black surround and post-deflection-acceleration tubes.

Let it be assumed that the tube under process is rectangular, even though it may alternatively be circular, and

has a diagonal dimension of the currently popular 23-inch size. The envelope of such a tube is initially formed of two major parts which are the faceplate section and the cone or funnel section. It is sufficient here to consider only the faceplate section which is rather in the form of a flanged i mitting its ready removal as required in various processing steps of the tube.

, The aperture mask is usually a subassembly of two principal components although it may be made as One integral structure. In either event, it has a mask portion having a multiplicity of apertures arranged, for the case under consideration, in a rectangular pattern and this maskis secured to a frame portion which has at least three and sometimes four leaf type mounting springs with apertures in their free ends to engage the mounting studs of the faceplate when it is desired to install the mask subassembly in position.

To screen the tube, the mask is used in a printing process through which interlaced patterns of different phosphor materials are established over the screen area and these patterns individually comprise a like multiplicity of phosphor dots having a diameter, approximately equal to, and arranged in the same pattern as, the apertures of the mask. Of the variety of printing schemes available, the most attractive ones include photoresist screening, electron printing and electrostatic printing. Since the invention does not lie inthe specifics of the printing technique, only the photo-etch process will be considered further.

A slurry is prepared which has the property of changing its solubility in a solvent upon exposure to actinic en'- ergy, preferably becoming insoluble in response to such exposure. A very convenient material for the slurry is polyvinyl alcohol sensitized with ammonium dichromate and, of course, a given phosphor material to be applied to the screen is included as an ingredient. In applying any given phosphor, such as the green phosphor, the green 'slurry is coated over the entire screen area and thereafter the mask is installed into the faceplate section in juxtaposition to the screen. At this time the mask, which is also essentially a spherical section, has its hole diameter precisely dimensioned for screening purposes and the faceplate with the mask are positioned in an exposure chamber or lighthouse. A light source positioned within that chamber to simulate the electron gun of the tube under process that is to be assigned to energize green phosphor is ener- .4 layer of light reflecting and conductive material, prefer.- ably, aluminum.

Having finished with the use of the mask in screening, it is necessary now to each the mask to increase the diameter of its apertures, relative to the diameter of the phosphor dots, to some desired value A Since the apertures are initially formed in the mask blank by an etching process, the etching now under consideration may be referred to as re-etch or etch back. It entails introducing the mask into an etch bath and opening the holes of the mask by etching with the parameters of the etch back step adjusted to enlarge the apertures the desired amount. In practicing this re-etch step, it is customary to first introduce the mask into a stripping solution which removes a blackening or The only other change is in the positioning of the light 7 source during exposure because each exposure takes place with the light source simulating the electron gun of the tube intended to excite the color phosphor currently being applied. At this point in the screening process, interlaced patterns of red, green and blue phosphor materials will oxidation layer that had previously been applied to the mask in order to avoid undesired reflections from the mask during the exposure steps. When the mask, stripped of its blackening layer, is introduced into the etch bath, material is removed concurrently from the walls defining the apertures and also the flat surfaces of the mask and it is apparent, therefore, that the ultimate thickness T of the mask blank varies with the etching time required to attain the desired final diameter A of the mask holes.

It will further be apparent, therefore, that the mechanical strength of the mask is dependent upon both of these dimensions. Their influences, however, are not the same as reflected by the curves of FIGS. 1 and 2. FIG. 1 has a full line curve which shows the change in mechanical strength with various values of final hole diameter A assuming a fixed final thickness T of the mask blank. The dotted curve of FIG. 1 is a plot of the same parameters and the two curves dilfer only in the specifics of making a determination of strength. For the full line curve the strength is a measure of the force required to collapse the mask by pressing at a corner of the mask frame with the mask supported in position within the faceplate section of the tube. The broken line curve is attained by a drop test and indicates the height required to cause the mask to collapse when dropped while supported in position within the faceplate.

FIG. 2 has a similar pair of curves, one of the full line and the other of broken line construction, taken for various final thicknesses T of the mask blank but with a given final aperture diameter A It is apparent, that the strength of the mask is much more severely affected by changes in mask thickness than final aperture size.

The curves of FIG. 3 are a plot of the quantityP/A versus mask strength and again two conditions are represented with the full line curve showing the results of the force test and the broken line curve showing the results of the drop test. The curves are very. similar and show the correlation and validity of the two different measuring procedures. The curves are non-linear, having a knee at a certain value of the quantity T /A and from the knee they approach a larger value asymptotically for increasing values of this quantity. In accordance with the subject invention, the etch back process is improved by dimensioning the thickness T of the mask and determining the parameters of the re-etch step to obtain for the quantity T /A a minimum value which occurs at the knee of the curve. For the specific case illustrated in FIG. 3, the knee of each curve is approximately at the abscissa value 13. It, of course, will be appreciated that the curves of FIG. 3 are obtained for a particular mask, namely, that of the 23-inch rectangular tube for which the mask has a radius of approximately 34.5 inches. For other tube sizes the curve, while having the same general shape, will be of specifically different values. Accordingly, the invention teaches that for the illustrative 23-inch tube the quantity T /A should have a minimum value of approximately 13 mils This corresponds to a final mask thickness T of approximately 5.95 mils, assuming that the final hole diameter of the mask is 16 mils. The abscissa point 18 1s a plot of mechanical strength corresponding to that of the conventional 23-inch shadow mask tube which has not been treated by a re-etch process, that is to say, the conventional tube in which the apertures of the mask are smaller than the phosphor dots. For the re-etch mask to exhibit the same mechanical strength, as the more conventional mask, and assuming the same mask aperture of 16 mils, a final mask thickness of 6.74 mils is required.

Control of the operating point of the process in respect of the curves of FIG. 3 may be exercised, as indicated above, by dimensioning the thickness of the mask blank and by determining the parameters of the re-etch step. It will be clear, by way of illustration, that if all other parameters remain fixed, the value of the final thickness T of the mask may be increased by increasing the weight or thickness of the material from which the blank is initially formed. Determination of the operating point with respect to the curves of FIG. 3, so far as the etch :bath is concerned, is conveniently possible by adjusting such known parameters as the concentration of the etchant which is usually ferric chloride where the mask is composed of cold rolled steel, adjusting the temperature of the bath and by control of the amount of etchant applied to the mask in the etching process. Usually, reetching is accomplished by directing the etching solution from a bank of spray heads to either or both sides of the blank and it is preferred that the pattern of etching solution be essentially uniform over the blank. An obvious control that is available is the rate of flow of etching solution delivered through the spray fixtures to the blank surface.

As the curves of FIG. 3 make clear, for operation below the knee of the curves a condition may be experienced in which the mask is so weakened during re-etch as to have inadequate mechanical strength, resulting in collapse and total destruction of the tube of which it is a part. The knee of the curve is a critical point below which it is not practicable to operate since this is a condition of minimal acceptable strength. Operation at any point above the knee will of course improve the strength up to a certain point although once the knee of the curve has been passed, the added strength, measured in cost of processing time, is not attractive.

6 I claim:

1. In the processing of a color cathode-ray tube, having an image screen and an aperture mask of etchable material provided with a multiplicity of apertures arranged in a predetermined pattern, which includes the steps of (a) utilizing said mask to print on said screen patterns of difierent phosphor materials individually comprising a like multiplicity of phosphor dots having a diameter approximately equal to, and arranged in the same pattern as, said apertures of said mask; and (b) thereafter back etching said mask to enlarge the apertures thereof to a dimension A the improvement which comprises controlling at least one parameter of said back etching step to obtain for the quantity T /A a value which occurs substantially at the knee of a curve relating said quantity to the mechanical strength of the etched mask, where T is the thickness of said mask after said etching step and said knee of said curve encompasses a range of values extending approximately from 12 mils 2 to 20 mils 2. The improvement, in accordance with claim 1, in the processing of a rectangular color tube having a diagonal screen dimension of about 23 inches, having a mask formed as a section of a sphere with a radius of about 34.5 inches in which said quantity T /A has a value of approximately 13 mils References Cited UNITED STATES PATENTS I 9/1956 Mears 1568 OTHER REFERENCES JACOB H. STE-INBERG, Primary Examiner US. Cl. X.R. 

